A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Development of new apparatus and methods in lithography have led to improvements in resolution of the imaged features, such as lines and contact holes or vias, patterned on a substrate, possibly leading to a resolution of less than 50 nm. This may be accomplished, for example, using relatively high numerical aperture (NA) projection systems (greater than 0.75 NA), a wavelength of 193 nm or less, and a plethora of techniques such as phase shift masks, non-conventional illumination and advanced photoresist processes.
However, certain small features such as contact holes are especially difficult to fabricate. The success of manufacturing processes at sub-wavelength resolutions will rely on the ability to print low modulation images or the ability to increase the image modulation to a level that will give acceptable lithographic yield.
Typically, the industry has used the Rayleigh criterion to evaluate the critical dimension (CD) and depth of focus (DOF) capability of a process. The CD and DOF measures can be given by the following equations:CD=k1(λ/ NA),  (1)andDOF=k2(λ/ NA2),  (2)where λ is the wavelength of the illumination radiation, k1 and k2 are constants for a specific lithographic process, and NA is the numerical aperture.
Additional measures that provide insight into the difficulties associated with lithography at the resolution limit include the Exposure Latitude (EL), the Dense:Isolated Bias (DIB), and the Mask Error Enhancement Factor (MEEF). The exposure latitude describes the percentage dose range where the printed pattern's critical dimension (CD) is within acceptable limits. For example, the exposure latitude may be defined as the change in exposure dose that causes a 10% change in printed line width. Exposure Latitude is a measure of reliability in printing features in lithography. It is used along with the DOF to determine the process window, i.e., the regions of focus and exposure that keep the final resist profile within prescribed specifications. Dense:Isolated Bias (also known as iso-dense bias) is a measure of the size difference between similar features, depending on the pattern density. The MEEF describes how patterning device CD errors are transmitted into substrate CD errors. Other imaging factors that may be taken into account include the pitch. The pitch is a distance between two features such as, for example, contact holes. In a simplified approximation of coherent illumination, the resolution of a lithography system may also be quoted in terms of the smallest half-pitch of a grating that is resolvable as a function of wavelength and numerical aperture NA.
Due to, among other things, variations in exposure and focus, patterns developed by lithographic processes are continually monitored or measured to determine if the dimensions of the patterns are within an acceptable range or to qualify the CD-uniformity (CDU). Monitoring of pattern features and measurement of its dimensions (metrology) is typically performed using either a scanning electron microscope (SEM) or an optical tool. Conventional SEM metrology has very high resolving power and is capable of resolving features of the order of 0.1 micron. However, SEM metrology is expensive to implement, relatively slow in operation and difficult to automate.
Measurements of CD are becoming increasingly challenging with the shrinking dimension of the device. As dimensions of the devices are becoming smaller, the margin of errors in CD of the devices are also decreasing, hence, requiring tighter process windows. As a result, there is a need for a method that would allow the user to extend the CD metrology to the next generation of device fabrication.